The present invention relates to an optical head apparatus and, more particularly, to the optics of an optical head apparatus for high storage density recording.
Optical disks have recording densities higher by about one order of magnitude than magnetic disks and other like memory devices. Removability is one distinct feature of optical disks that are capable of storing not only numbers and characters but also images and audio signals. As they accommodate such data in large quantities, optical disks are expected to play a major role from now on as a large-capacity storage memory addressing multimedia applications.
In the field of laser technology, improvements have been made to use laser sources of progressively shorter wavelengths: from conventional infrared rays to red rays having wavelengths on the order of 600 nm, green rays on the order of 500 nm, blue rays on the order of 400 nm, purple rays on the order of 300 nm, and ultraviolet rays. The trend toward shorter wavelengths is to make recording densities higher than ever before. One highly promising short wavelength laser source reported recently is a II-VI compound diode laser having a band gap wider than the conventional III-V compound diode laser. An alternative and similarly energetic approach to shortening the wavelength of the laser source centers on the use of wavelength conversion based on nonlinear optics.
Aside from the attempts to enhance the recording density of optical disks using laser sources of shorter wavelengths, some researchers are trying to increase the numerical aperture (NA) of objective lenses. The spot size of a light beam emitted by a laser source on an optical disk surface is given by dividing the wavelength of the beam by the numerical aperture (NA) of the objective lens in use. This is because recording density is inversely proportional to the squared reciprocal number of the spot size involved.
Larger numerical apertures (NA) of the objective lens entail some disadvantages: a pronounced coma aberration or astigmatic aberration relative to an objective lens tilt or an optical disk tilt. That bottleneck leads to a drop in the Strehl intensity of the light spot focused on the optical disk surface. With the spot size getting larger and with the resolution power reduced correspondingly, a readout signal from the optical disk may fail to reach a predetermined tolerance level. This makes it impossible to read the optical disk. In terms of wave aberration, coma aberration increases in proportion to the numerical aperture cubed, and astigmatic aberration grows in proportion to the numerical aperture squared. That is, the greater the numerical aperture of the objective lens for recording of higher densities, the higher the increase in aberration. Eventually the optical disk becomes impossible to read.
One conventional solution to the above drawback is what is known as a tilt servo mechanism, adopted in addition to the auto focusing and the tracking servo mechanism of the optical head regarding the optical disk. The tilt servo mechanism is a servo mechanism that keeps aligning the objective lens axis with the normal direction of the optical disk surface. But this mechanism has some disadvantages of its own. Because the tilt servo mechanism has a three-dimensional actuator implemented by adding a tilting actuator to the traditional two-dimensional actuator, it has a more complicated structure. That in turn means a heavier weight and higher costs. In view of today's trends toward preferring downsized information systems, this solution has failed to gain widespread popularity.
Meanwhile, as disclosed by Hiroshi Kubota et al. in "Handbook for Optics" (Asakura Shoten Co., 12th print, supplemented edition, 1990, pp. 180-181), the imaging quality of images formed on a camera film is reportedly improved by furnishing an aberration correction filter in the pupil position of an aberration-containing imaging lens such as a camera lens. Attempts have been made to apply this method to the optics of steppers for manufacturing semiconductor LSI chips, as disclosed in the Micro-Process Conference '93 digest A-4-2, pp. 40-41. However, the optics cited above was intended to improve the imaging quality of two-dimensional images, either on the film with the camera lens or on the wafer with the stepper lens. As such, the cited optics is not relevant to applications dealt with by this invention, i.e., applications involving the construction of the laser beam focusing spot for the optical head apparatus. New measures are thus needed to install the aberration correction filter in the pupil position.